It is universally known that a europium-activated yttrium oxide fluorescent material (Y.sub.2 O.sub.3 :Eu) excels in such fluorescent properties as brightness and color of the luminescence excited by an electron beam and an ultraviolet ray and, owing to this feature, finds utility as a red light component fluorescent material for color cathode ray tubes and as a red light component fluorescent material for three band type fluorescent lamps.
Various fluxes have been heretofore studied with a view to enabling such europium-activated rare earth oxide fluorescent material as mentioned above to acquire an exalted luminescent efficiency. For example, such compounds as lithium phosphate (refer to JP-A-56-99,276), borates of alkaline earth metals (refer to JP-A-55-161,883), aluminum phosphate (refer to JP-A-58-52,382), barium borate (refer to JP-A-59-45,384), oxides of alkaline earth metals (refer to JP-A-58-127,777), and barium fluoride.magnesium fluoride (refer to JP-A-61-266,488) have been proposed as fluxes for the europium-activated yttrium oxide fluorescent material.
The fluorescent material using such a flux as mentioned above acquires a conspicuous increase in crystal size and the fluorescent lamp using this fluorescent material enjoys an exalted initial luminescent efficiency. For the reason shown hereinbelow, however, the fluorescent lamp entails the problem that the luminescent efficiency thereof gradually dwindles with the elapse of the lighting time thereof.
The process for the manufacture (synthesis) of an ordinary fluorescent material includes a step of washing. The conventional flux is normally removed at this step of washing. In the case of the fluorescent material which is produced with such a flux as mentioned above, the decline of the luminescent efficiency with the elapse of the lighting time of the lamp may be logically explained by a postulate that a minute amount of this flux will remain therein after the washing step and the residual flux will gradually react with such component substances of the fluorescent lamp as glass and mercury.
When the red light fluorescent material has the luminescent efficiency thereof degraded by aging as described above, this degradation results in a decline of the luminescent brightness for the fluorescent lamp. In the three band type fluorescent lamp, the degradation entails an alteration of the luminescent chromaticity and inevitably impairs the quality of the fluorescent lamp seriously.
An object of this invention is to provide a red light fluorescent material which possesses high initial luminescent brightness (efficiency) and causes only a small degradation in the efficiency of the fluorescent lamp while in use. Another object of this invention is to provide a fluorescent lamp which uses this fluorescent material and consequently manifests high color-rendering property and high luminescent brightness while repressing the alteration of luminescent brightness and luminescent chromaticity by aging.